Apparent magnitude (m) is the brightness of a star or other astronomical object observed from the Earth. An object's apparent magnitude depends on its intrinsic luminosity, its distance from Earth, and any extinction of the object's light by interstellar dust along the line of sight to the observer.
The magnitude scale is reverse logarithmic. An object with a magnitude 5 higher than another's magnitude is 100 times dimmer. Consequently, a difference of 1.0 in magnitude corresponds to a brightness ratio of 5√ or about 2.512. The brighter an object is, the lower its magnitude. For example, a star of magnitude 2.0 is 2.512 times brighter than a star of magnitude 3.0, or 100 times brighter than one of magnitude 7.0. The brightest astronomical objects have negative apparent magnitudes: for example, Venus at −4.2 or Sirius at −1.46. The faintest naked-eye stars visible on the darkest night have apparent magnitudes of about +6.5. The apparent magnitudes of known objects range from the Sun at −26.7 to objects in deep Hubble Space Telescope images of around magnitude +30.
The measurement of apparent magnitude is called photometry. Photometric measurements are made in the ultraviolet, visible, or infrared wavelength bands using standard passband filters belonging to photometric systems such as the UBV system or the Strömgren uvbyβ system.
Absolute magnitude is the intrinsic luminosity rather than the apparent brightness of a celestial object, expressed on the same reverse logarithmic scale. Absolute magnitude is defined as the apparent magnitude that a star or object would have if it were observed from a distance of 10 parsecs. When referring to just "magnitude", apparent magnitude rather than absolute magnitude is normally intended.
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|Number of stars |
(other than Sun)
in the night sky
The scale used to indicate magnitude originates in the Hellenistic practice of dividing stars visible to the naked eye into six magnitudes. The brightest stars in the night sky were said to be of first magnitude (m = 1), whereas the faintest were of sixth magnitude (m = 6), which is the limit of human visual perception (without the aid of a telescope). Each grade of magnitude was considered twice the brightness of the following grade (a logarithmic scale), although that ratio was subjective as no photodetectors existed. This rather crude scale for the brightness of stars was popularized by Ptolemy in his Almagest and is generally believed to have originated with Hipparchus.
In 1856, Norman Robert Pogson formalized the system by defining a first magnitude star as a star that is 100 times as bright as a sixth-magnitude star, thereby establishing the logarithmic scale still in use today. This implies that a star of magnitude m is about 2.512 times as bright as a star of magnitude m + 1. This figure, the fifth root of 100, became known as Pogson's Ratio. The zero point of Pogson's scale was originally defined by assigning Polaris a magnitude of exactly 2. Astronomers later discovered that Polaris is slightly variable, so they switched to Vega as the standard reference star, assigning the brightness of Vega as the definition of zero magnitude at any specified wavelength.
Apart from small corrections, the brightness of Vega still serves as the definition of zero magnitude for visible and near infrared wavelengths, where its spectral energy distribution (SED) closely approximates that of a black body for a temperature of 11000 K. However, with the advent of infrared astronomy it was revealed that Vega's radiation includes an infrared excess presumably due to a circumstellar disk consisting of dust at warm temperatures (but much cooler than the star's surface). At shorter (e.g. visible) wavelengths, there is negligible emission from dust at these temperatures. However, in order to properly extend the magnitude scale further into the infrared, this peculiarity of Vega should not affect the definition of the magnitude scale. Therefore, the magnitude scale was extrapolated to all wavelengths on the basis of the black-body radiation curve for an ideal stellar surface at 11000 K uncontaminated by circumstellar radiation. On this basis the spectral irradiance (usually expressed in janskys) for the zero magnitude point, as a function of wavelength, can be computed. Small deviations are specified between systems using measurement apparatuses developed independently so that data obtained by different astronomers can be properly compared, but of greater practical importance is the definition of magnitude not at a single wavelength but applying to the response of standard spectral filters used in photometry over various wavelength bands.
With the modern magnitude systems, brightness over a very wide range is specified according to the logarithmic definition detailed below, using this zero reference. In practice such apparent magnitudes do not exceed 30 (for detectable measurements). The brightness of Vega is exceeded by four stars in the night sky at visible wavelengths (and more at infrared wavelengths) as well as the bright planets Venus, Mars, and Jupiter, and these must be described by negative magnitudes. For example, Sirius, the brightest star of the celestial sphere, has a magnitude of −1.4 in the visible. Negative magnitudes for other very bright astronomical objects can be found in the table below.
Astronomers have developed other photometric zeropoint systems as alternatives to the Vega system. The most widely used is the AB magnitude system,in which photometric zeropoints are based on a hypothetical reference spectrum having constant flux per unit frequency interval, rather than using a stellar spectrum or blackbody curve as the reference. The AB magnitude zeropoint is defined such that an object's AB and Vega-based magnitudes will be approximately equal in the V filter band.
Precision measurement of magnitude (photometry) requires calibration of the photographic or (usually) electronic detection apparatus. This generally involves contemporaneous observation, under identical conditions, of standard stars whose magnitude using that spectral filter is accurately known. Moreover, as the amount of light actually received by a telescope is reduced due to transmission through the Earth's atmosphere, the airmasses of the target and calibration stars must be taken into account. Typically one would observe a few different stars of known magnitude which are sufficiently similar. Calibrator stars close in the sky to the target are favoured (to avoid large differences in the atmospheric paths). If those stars have somewhat different zenith angles (altitudes) then a correction factor as a function of airmass can be derived and applied to the airmass at the target's position. Such calibration obtains the brightnesses as would be observed from above the atmosphere, where apparent magnitude is defined.
The dimmer an object appears, the higher the numerical value given to its magnitude, with a difference of 5 magnitudes corresponding to a brightness factor of exactly 100. Therefore, the magnitude m, in the spectral band x, would be given by
which is more commonly expressed in terms of common (base-10) logarithms as
where Fx is the observed flux density using spectral filter x, and Fx,0 is the reference flux (zero-point) for that photometric filter. Since an increase of 5 magnitudes corresponds to a decrease in brightness by a factor of exactly 100, each magnitude increase implies a decrease in brightness by the factor 5√ ≈ 2.512 (Pogson's ratio). Inverting the above formula, a magnitude difference m1 − m2 = Δm implies a brightness factor of
What is the ratio in brightness between the Sun and the full Moon?
The apparent magnitude of the Sun is −26.74 (brighter), and the mean magnitude of the full moon is −12.74 (dimmer).
Difference in magnitude:
The Sun appears about 400000 times brighter than the full moon.
Sometimes one might wish to add brightnesses. For example, photometry on closely separated double stars may only be able to produce a measurement of their combined light output. How would we reckon the combined magnitude of that double star knowing only the magnitudes of the individual components? This can be done by adding the brightnesses (in linear units) corresponding to each magnitude.
Solving for yields
where mf is the resulting magnitude after adding the brightnesses referred to by m1 and m2.
While magnitude generally refers to a measurement in a particular filter band corresponding to some range of wavelengths, the apparent or absolute bolometric magnitude (mbol) is a measure of an object's apparent or absolute brightness integrated over all wavelengths of the electromagnetic spectrum (also known as the object's irradiance or power, respectively). The zeropoint of the apparent bolometric magnitude scale is based on the definition that an apparent bolometric magnitude of 0 mag is equivalent to a received irradiance of 2.518×10−8 W·m−2 (Watts per square metre.)
While apparent magnitude is a measure of the brightness of an object as seen by a particular observer, absolute magnitude is a measure of the intrinsic brightness of an object. Flux decreases with distance according to an inverse-square law, so the apparent magnitude of a star depends on both its absolute brightness and its distance (and any extinction). For example, a star at one distance will have the same apparent magnitude as a star four times brighter at twice that distance. In contrast, the intrinsic brightness of an astronomical object, does not depend on the distance of the observer or any extinction.
The absolute magnitude M, of a star or astronomical object is defined as the apparent magnitude it would have as seen from a distance of 10 parsecs (about 32.6 light-years). The absolute magnitude of the Sun is 4.83 in the V band (green) and 5.48 in the B band (blue).
In the case of a planet or asteroid, the absolute magnitude H rather means the apparent magnitude it would have if it were 1 astronomical unit from both the observer and the Sun, and fully illuminated (a configuration that is only theoretically achievable, with the observer situated on the surface of the Sun).[ citation needed ]
|Flux at m = 0, Fx,0|
The magnitude scale is a reverse logarithmic scale. A common misconception is that the logarithmic nature of the scale is because the human eye itself has a logarithmic response. In Pogson's time this was thought to be true (see Weber–Fechner law), but it is now believed that the response is a power law (see Stevens' power law).
Magnitude is complicated by the fact that light is not monochromatic. The sensitivity of a light detector varies according to the wavelength of the light, and the way it varies depends on the type of light detector. For this reason, it is necessary to specify how the magnitude is measured for the value to be meaningful. For this purpose the UBV system is widely used, in which the magnitude is measured in three different wavelength bands: U (centred at about 350 nm, in the near ultraviolet), B (about 435 nm, in the blue region) and V (about 555 nm, in the middle of the human visual range in daylight). The V band was chosen for spectral purposes and gives magnitudes closely corresponding to those seen by the human eye. When an apparent magnitude is discussed without further qualification, the V magnitude is generally understood.[ citation needed ]
Because cooler stars, such as red giants and red dwarfs, emit little energy in the blue and UV regions of the spectrum their power is often under-represented by the UBV scale. Indeed, some L and T class stars have an estimated magnitude of well over 100, because they emit extremely little visible light, but are strongest in infrared.[ citation needed ]
Measures of magnitude need cautious treatment and it is extremely important to measure like with like. On early 20th century and older orthochromatic (blue-sensitive) photographic film, the relative brightnesses of the blue supergiant Rigel and the red supergiant Betelgeuse irregular variable star (at maximum) are reversed compared to what human eyes perceive, because this archaic film is more sensitive to blue light than it is to red light. Magnitudes obtained from this method are known as photographic magnitudes, and are now considered obsolete.[ citation needed ]
For objects within the Milky Way with a given absolute magnitude, 5 is added to the apparent magnitude for every tenfold increase in the distance to the object. For objects at very great distances (far beyond the Milky Way), this relationship must be adjusted for redshifts and for non-Euclidean distance measures due to general relativity.
For planets and other Solar System bodies the apparent magnitude is derived from its phase curve and the distances to the Sun and observer.[ citation needed ]
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|−67.57||gamma-ray burst GRB 080319B||seen from 1 AU away|
|−40.07||star Zeta1 Scorpii||seen from 1 AU away|
|−39.66||star R136a1||seen from 1 AU away|
|−38.00||star Rigel||seen from 1 AU away||It would be seen as a large very bright bluish disk of 35° apparent diameter.|
|−30.30||star Sirius A||seen from 1 AU away|
|−29.30||star Sun||seen from Mercury at perihelion|
|−27.40||star Sun||seen from Venus at perihelion|
|−26.74||star Sun||seen from Earth||About 400,000 times brighter than mean full moon[ citation needed ]|
|−25.60||star Sun||seen from Mars at aphelion|
|−25.00||Minimum brightness that causes the typical eye slight pain to look at|
|−23.00||star Sun||seen from Jupiter at aphelion|
|−21.70||star Sun||seen from Saturn at aphelion|
|−20.20||star Sun||seen from Uranus at aphelion|
|−19.30||star Sun||seen from Neptune|
|−18.20||star Sun||seen from Pluto at aphelion|
|−16.70||star Sun||seen from Eris at aphelion|
|−14.20||An illumination level of 1 lux|
|−12.90||full moon||seen from Earth at perihelion||maximum brightness of perigee + perihelion + full moon (mean distance value is −12.74, though values are about 0.18 magnitude brighter when including the opposition effect)|
|−12.40||Betelgeuse||seen from Earth when it goes supernova|
|−11.20||star Sun||seen from Sedna at aphelion|
|−10.00||Comet Ikeya–Seki (1965)||seen from Earth||which was the brightest Kreutz Sungrazer of modern times|
|−9.50||Iridium (satellite) flare||seen from Earth||maximum brightness|
|−7.50||supernova of 1006||seen from Earth||the brightest stellar event in recorded history (7200 light-years away)|
|−6.50||The total integrated magnitude of the night sky||seen from Earth|
|−6.00||Crab Supernova of 1054||seen from Earth||(6500 light-years away)|
|−5.90||International Space Station||seen from Earth||when the ISS is at its perigee and fully lit by the Sun|
|−4.92||planet Venus||seen from Earth||maximum brightness when illuminated as a crescent|
|−4.14||planet Venus||seen from Earth||mean brightness|
|−4||Faintest objects observable during the day with naked eye when Sun is high|
|−3.99||star Epsilon Canis Majoris||seen from Earth||maximum brightness of 4.7 million years ago, the historical brightest star of the last and next five million years|
|−2.98||planet Venus||seen from Earth||minimum brightness when it is on the far side of the Sun|
|−2.94||planet Jupiter||seen from Earth||maximum brightness|
|−2.94||planet Mars||seen from Earth||maximum brightness|
|−2.5||Faintest objects visible during the day with naked eye when Sun is less than 10° above the horizon|
|−2.50||new moon||seen from Earth||minimum brightness|
|−2.48||planet Mercury||seen from Earth||maximum brightness at superior conjunction (unlike Venus, Mercury is at its brightest when on the far side of the Sun, the reason being their different phase curves)|
|−2.20||planet Jupiter||seen from Earth||mean brightness|
|−1.66||planet Jupiter||seen from Earth||minimum brightness|
|−1.47||star system Sirius||seen from Earth||Brightest star except for the Sun at visible wavelengths|
|−0.83||star Eta Carinae||seen from Earth||apparent brightness as a supernova impostor in April 1843|
|−0.72||star Canopus||seen from Earth||2nd brightest star in night sky|
|−0.55||planet Saturn||seen from Earth||maximum brightness near opposition and perihelion when the rings are angled toward Earth|
|−0.3||Halley's comet||seen from Earth||Expected apparent magnitude at 2061 passage|
|−0.27||star system Alpha Centauri AB||seen from Earth||Combined magnitude (3rd brightest star in night sky)|
|−0.04||star Arcturus||seen from Earth||4th brightest star to the naked eye|
|−0.01||star Alpha Centauri A||seen from Earth||4th brightest individual star visible telescopically in the night sky|
|+0.03||star Vega||seen from Earth||which was originally chosen as a definition of the zero point|
|+0.23||planet Mercury||seen from Earth||mean brightness|
|+0.50||star Sun||seen from Alpha Centauri|
|+0.46||planet Saturn||seen from Earth||mean brightness|
|+0.71||planet Mars||seen from Earth||mean brightness|
|+1.17||planet Saturn||seen from Earth||minimum brightness|
|+1.86||planet Mars||seen from Earth||minimum brightness|
|+3.03||supernova SN 1987A||seen from Earth||in the Large Magellanic Cloud (160,000 light-years away)|
|+3 to +4||Faintest stars visible in an urban neighborhood with naked eye|
|+3.44||Andromeda Galaxy||seen from Earth||M31|
|+4||Orion Nebula||seen from Earth||M42|
|+4.38||moon Ganymede||seen from Earth||maximum brightness (moon of Jupiter and the largest moon in the Solar System)|
|+4.50||open cluster M41||seen from Earth||an open cluster that may have been seen by Aristotle|
|+4.5||Sagittarius Dwarf Spheroidal Galaxy||seen from Earth|
|+5.20||asteroid Vesta||seen from Earth||maximum brightness|
|+5.38||planet Uranus||seen from Earth||maximum brightness|
|+5.68||planet Uranus||seen from Earth||mean brightness|
|+5.72||spiral galaxy M33||seen from Earth||which is used as a test for naked eye seeing under dark skies|
|+5.8||gamma-ray burst GRB 080319B||seen from Earth||Peak visual magnitude (the "Clarke Event") seen on Earth on March 19, 2008 from a distance of 7.5 billion light-years.|
|+6.03||planet Uranus||seen from Earth||minimum brightness|
|+6.49||asteroid Pallas||seen from Earth||maximum brightness|
|+6.5||Approximate limit of stars observed by a mean naked eye observer under very good conditions. There are about 9,500 stars visible to mag 6.5.|
|+6.64||dwarf planet Ceres||seen from Earth||maximum brightness|
|+6.75||asteroid Iris||seen from Earth||maximum brightness|
|+6.90||spiral galaxy M81||seen from Earth||This is an extreme naked-eyetarget that pushes human eyesight and the Bortle scale to the limit|
|+7 to +8||Extreme naked-eye limit, Class 1 on Bortle scale, the darkest skies available on Earth|
|+7.25||planet Mercury||seen from Earth||minimum brightness|
|+7.67||planet Neptune||seen from Earth||maximum brightness|
|+7.78||planet Neptune||seen from Earth||mean brightness|
|+8.00||planet Neptune||seen from Earth||minimum brightness|
|+8.10||moon Titan||seen from Earth||maximum brightness; largest moon of Saturn; mean opposition magnitude 8.4|
|+8.29||star UY Scuti||seen from Earth||Maximum brightness; largest known star by radius|
|+8.94||asteroid 10 Hygiea||seen from Earth||maximum brightness|
|+9.50||Faintest objects visible using common 7×50 binoculars under typical conditions|
|+10.20||moon Iapetus||seen from Earth||maximum brightness, brightest when west of Saturn and takes 40 days to switch sides|
|+10.7||Luhman 16||seen from Earth||Closest brown dwarfs|
|+11.05||star Proxima Centauri||seen from Earth||2nd closest star|
|+11.8||moon Phobos||seen from Earth||Maximum brightness; brightest moon of Mars|
|+12.23||star R136a1||seen from Earth||Most luminous and massive star known|
|+12.89||moon Deimos||seen from Earth||Maximum brightness|
|+12.91||quasar 3C 273||seen from Earth||brightest (luminosity distance of 2.4 billion light-years)|
|+13.42||moon Triton||seen from Earth||Maximum brightness|
|+13.65||dwarf planet Pluto||seen from Earth||maximum brightness, 725 times fainter than magnitude 6.5 naked eye skies|
|+13.9||moon Titania||seen from Earth||Maximum brightness; brightest moon of Uranus|
|+14.1||star WR 102||seen from Earth||Hottest known star|
|+15.4||centaur Chiron||seen from Earth||maximum brightness|
|+15.55||moon Charon||seen from Earth||maximum brightness (the largest moon of Pluto)|
|+16.8||dwarf planet Makemake||seen from Earth||Current opposition brightness|
|+17.27||dwarf planet Haumea||seen from Earth||Current opposition brightness|
|+18.7||dwarf planet Eris||seen from Earth||Current opposition brightness|
|+19.5||Faintest objects observable with the Catalina Sky Survey 0.7-meter telescope using a 30 second exposure|
|+20.7||moon Callirrhoe||seen from Earth||(small ≈8 km satellite of Jupiter)|
|+22||Faintest objects observable in visible light with a 600 mm (24″) Ritchey-Chrétien telescope with 30 minutes of stacked images (6 subframes at 5 minutes each) using a CCD detector|
|+22.91||moon Hydra||seen from Earth||maximum brightness of Pluto's moon|
|+23.38||moon Nix||seen from Earth||maximum brightness of Pluto's moon|
|+24||Faintest objects observable with the Pan-STARRS 1.8-meter telescope using a 60 second exposure|
|+25.0||moon Fenrir||seen from Earth||(small ≈4 km satellite of Saturn)|
|+27.7||Faintest objects observable with a single 8-meter class ground-based telescope such as the Subaru Telescope in a 10-hour image|
|+28.2||Halley's Comet||seen from Earth||in 2003 when it was 28 AU from the Sun, imaged using 3 of 4 synchronised individual scopes in the ESO's Very Large Telescope array using a total exposure time of about 9 hours|
|+28.4||asteroid 2003 BH91||seen from Earth orbit||observed magnitude of ≈15-kilometer Kuiper belt object Seen by the Hubble Space Telescope (HST) in 2003, dimmest known directly-observed asteroid.|
|+31.5||Faintest objects observable in visible light with Hubble Space Telescope via the EXtreme Deep Field with ~23 days of exposure time collected over 10 years|
|+34||Faintest objects observable in visible light with James Webb Space Telescope|
|+35||unnamed asteroid||seen from Earth orbit||expected magnitude of dimmest known asteroid, a 950-meter Kuiper belt object discovered by the HST passing in front of a star in 2009.|
|+35||star LBV 1806-20||seen from Earth||a luminous blue variable star, expected magnitude at visible wavelengths due to interstellar extinction|
Some of the listed magnitudes are approximate. Telescope sensitivity depends on observing time, optical bandpass, and interfering light from scattering and airglow.
Absolute magnitude is a measure of the luminosity of a celestial object, on an inverse logarithmic astronomical magnitude scale. An object's absolute magnitude is defined to be equal to the apparent magnitude that the object would have if it were viewed from a distance of exactly 10 parsecs, without extinction of its light due to absorption by interstellar matter and cosmic dust. By hypothetically placing all objects at a standard reference distance from the observer, their luminosities can be directly compared on a magnitude scale.
Corona Australis is a constellation in the Southern Celestial Hemisphere. Its Latin name means "southern crown", and it is the southern counterpart of Corona Borealis, the northern crown. It is one of the 48 constellations listed by the 2nd-century astronomer Ptolemy, and it remains one of the 88 modern constellations. The Ancient Greeks saw Corona Australis as a wreath rather than a crown and associated it with Sagittarius or Centaurus. Other cultures have likened the pattern to a turtle, ostrich nest, a tent, or even a hut belonging to a rock hyrax.
Caelum is a faint constellation in the southern sky, introduced in the 1750s by Nicolas Louis de Lacaille and counted among the 88 modern constellations. Its name means “chisel” in Latin, and it was formerly known as Caelum Scalptorium ; It is a rare word, unrelated to the far more common Latin caelum, meaning “sky, heaven, atmosphere”. It is the eighth-smallest constellation, and subtends a solid angle of around 0.038 steradians, just less than that of Corona Australis.
Luminosity is an absolute measure of radiated electromagnetic power (light), the radiant power emitted by a light-emitting object.
Andromeda is one of the 48 constellations listed by the 2nd-century Greco-Roman astronomer Ptolemy and remains one of the 88 modern constellations. Located north of the celestial equator, it is named for Andromeda, daughter of Cassiopeia, in the Greek myth, who was chained to a rock to be eaten by the sea monster Cetus. Andromeda is most prominent during autumn evenings in the Northern Hemisphere, along with several other constellations named for characters in the Perseus myth. Because of its northern declination, Andromeda is visible only north of 40° south latitude; for observers farther south, it lies below the horizon. It is one of the largest constellations, with an area of 722 square degrees. This is over 1,400 times the size of the full moon, 55% of the size of the largest constellation, Hydra, and over 10 times the size of the smallest constellation, Crux.
3C 273 is a quasar located in the constellation Virgo. It was the first quasar ever to be identified.
Photometry, from Greek photo- ("light") and -metry ("measure"), is a technique used in astronomy that is concerned with measuring the flux or intensity of light radiated by astronomical objects. This light is measured through a telescope using a photometer, often made using electronic devices such as a CCD photometer or a photoelectric photometer that converts light into an electric current by the photoelectric effect. When calibrated against standard stars of known intensity and colour, photometers can measure the brightness or apparent magnitude of celestial objects.
Circinus is a small, faint constellation in the southern sky, first defined in 1756 by the French astronomer Nicolas-Louis de Lacaille. Its name is Latin for compass, referring to the drafting tool used for drawing circles. Its brightest star is Alpha Circini, with an apparent magnitude of 3.19. Slightly variable, it is the brightest rapidly oscillating Ap star in the night sky. AX Circini is a Cepheid variable visible with the unaided eye, and BX Circini is a faint star thought to have been formed from the merger of two white dwarfs. Two sun-like stars have planetary systems: HD 134060 has two small planets, and HD 129445 has a Jupiter-like planet. Supernova SN 185 appeared in Circinus in 185 AD and was recorded by Chinese observers. Two novae have been observed more recently, in the 20th century.
Messier 14 is a globular cluster of stars in the constellation Ophiuchus. It was discovered by Charles Messier in 1764.
The Dumbbell Nebula is a planetary nebula in the constellation Vulpecula, at a distance of about 1227 light-years. This object was the first planetary nebula to be discovered; by Charles Messier in 1764. At its brightness of visual magnitude 7.5 and its diameter of about 8 arcminutes, it is easily visible in binoculars, and a popular observing target in amateur telescopes.
The cosmic distance ladder is the succession of methods by which astronomers determine the distances to celestial objects. A real direct distance measurement of an astronomical object is possible only for those objects that are "close enough" to Earth. The techniques for determining distances to more distant objects are all based on various measured correlations between methods that work at close distances and methods that work at larger distances. Several methods rely on a standard candle, which is an astronomical object that has a known luminosity.
The distance modulus is a way of expressing distances that is often used in astronomy. It describes distances on a logarithmic scale based on the astronomical magnitude system.
In astronomy, magnitude is a unitless measure of the brightness of an object in a defined passband, often in the visible or infrared spectrum, but sometimes across all wavelengths. An imprecise but systematic determination of the magnitude of objects was introduced in ancient times by Hipparchus.
Photographic magnitude is a measure of the relative brightness of a star or other astronomical object as imaged on a photographic film emulsion with a camera attached to a telescope. An object's apparent photographic magnitude depends on its intrinsic luminosity, its distance and any extinction of light by interstellar matter existing along the line of sight to the observer.
38 Aquarii is a star in the equatorial constellation of Aquarius. 38 Aquarii is its Flamsteed designation though it also bears the Bayer designation of e Aquarii. It is a faint star but visible to the naked eye, with an apparent visual magnitude of +5.43. The distance to this star, based upon parallax measurements, is around 450 light-years.
21 Aquilae is a solitary variable star in the equatorial constellation of Aquila. It has the variable star designation V1288 Aql; 21 Aquilae is its Flamsteed designation. This object is visible to the naked eye as a dim, blue-white hued star with a baseline apparent visual magnitude of 5.14. The star is located at a distance of around 740 light-years from Earth, give or take a 30 light-year margin of error. It is moving closer to the Earth with a heliocentric radial velocity of –5 km/s.
NGC 6302, also called the Bug Nebula or Butterfly Nebula, is a bipolar planetary nebula in the constellation Scorpius. The structure in the nebula is among the most complex ever observed in planetary nebulae. The spectrum of NGC 6302 shows that its central star is one of the hottest stars known, with a surface temperature in excess of 250,000 degrees Celsius, implying that the star from which it formed must have been very large.
Eta Crucis is a solitary star in the southern constellation of Crux. It can be seen with the naked eye, having an apparent visual magnitude of 4.14m. Based upon parallax measurements, η Crucis is located 64 light-years from the Sun. The system made its closest approach about 1.6 million years ago when it achieved perihelion at a distance of roughly 26 light years.
In astronomy, the bolometric correction is the correction made to the absolute magnitude of an object in order to convert its visible magnitude to its bolometric magnitude. It is large for stars which radiate much of their energy outside of the visible range. A uniform scale for the correction has not yet been standardized.
Mu Canis Majoris is a binary star system in the southern constellation of Canis Major. The pair can be located a little to the southwest of the point midway between Gamma and Theta Canis Majoris, and the components can be split with a small telescope. The system is faintly visible to the naked eye with an apparent visual magnitude of 5.12. Based upon an annual parallax shift of just 2.62 mas as seen from Earth, this system is located roughly 1,200 light years from the Sun.